I'm probably an unsuitable person to ask this since my TV is basically a glorified monitor, but from what I could tell there are no downsides except for the obvious one being that I can't update my TV. But considering much bloat and spying is going on with (LG) smart TVs, I don't mind it all.

I use Twitch, YouTube, and Jellyfin on mine, and it's been awesome. I hadn't used any other functionality or apps the TV comes with before rooting and I likely won't in the future. So even if some of them didn't work, I couldn't care less.

Maybe do some research and find out for your specific model?